Analog voltage comparators (comparators) are widely used building blocks in electronic circuits that have an input stage comprising a differential pair of MOS transistors which are designed to be closely matched to one another. For example, the comparator is the basic analog-to-digital interface element for wireless and very-large-scale integration (VLSI) systems, analog/mixed integrated circuits (ICs), analog-to-digital converters (ADC's) and digital-to-analog converters (DAC's).
A comparator can be an open loop device which compares a signal voltage (VIN) on one input with a reference voltage (ground) on its other input that generates a digital output being logic 1 or 0 when one of the signal levels is greater than the other. The comparator's external pins include a differential pair having a non-inverting input (+), an inverting input (−), and the comparator generally also includes an output pin. In some applications the comparator is a closed-loop device configured as a Schmitt trigger which is a comparator circuit with hysteresis that is implemented by applying positive feedback to the non-inverting input of the comparator.
In a variety of any applications such as for ADC's, during operations the comparator is subjected to dynamic signals that can include large input voltage (Vin) levels. In a conventional MOS comparator, the threshold voltage (Vt) of the input differential pair of MOS transistors limits the Vin range of the comparator. If the differential pair comprises NMOS transistors, the lower limit of the Vin range is the threshold voltage (Vt) of the NMOS transistor. If the differential pair uses PMOS transistors, the higher limit of the Vin range is VDD minus the Vt of the PMOS transistors.